In installations for air conditioning large buildings from a single thermal source it is customary to divide the total space into a number of zones, each provided with a thermostat and means actuated thereby for regulating the rate of flow of conditioned air from the source to the room so that it is heated or cooled to the desire temperature. Successful temperature control in a zone by this system assumes that the pressure at which conditioned air is supplied remains uniform, but uniform pressure is not easy to obtain in systems having numerous zones independently increasing or decreasing their loads on the source. To take care of this it has been proposed to provide flow responsive means, in addition to the usual temperature responsive means, for limiting the rate of air flow to each of the several zones. At least one such system has been devised in which the entire control operation is actuated fluidically, the control medium being air taken from the source itself. Such a system is shown in Osheroff U.S. Pat. No. 3,840,177, in which the flow of air from a source to a zone is varied by a bellows under the control of a fluidic device actuated in turn by a thermostat in the zone. In that system the rate of flow is limited by apparatus including a pressure probe which regulates the escape of air from the bellows.